Pinhole projector for viewing furnace interiors



M. FOGUEL 3,249,005

PINHOLE PROJECTOR FOR VIEWING FURNACE INTERIORS May 3, 1966 Filed Feb. 24, 1964 Fig.2

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INVENTOR.

BY Max Fogue! f .1

United States Patent 3,249,005 PIYHOLE PRQEECTOR FOR VEEWING FURNAE INTERIQRS Max Foguel, Jerusalem, Israel, assignor to Yissum Research Development Company, Jerusalem, lsrael, a company of Israel Filed Feb. 24, 1964, Ser. N 346,592 Claims priority, application Israel, Feb. 27, 1963, 18,794 5 Claims. (Cl. 88-24) This invention concerns an improved mufiie furnace. A specific object of this invention is to provide improve ments in mufiie furnaces adapted for the visual inspec tion of the rnufiie interior during operation without disturbing the thermal conditions inside the mufile.

Another object of this invention is to provide an lifl' proved mufile furnace adapted for growing single crystals by flame-fusion (Verneuil method) with visual inspection of the crystal during growth to determine its size, shape, position and color, without subjecting it to sharp thermal gradients.

Other and further objects of this invention will be come apparent hereinafter.

In the high temperature treatment of materials use is often made of muife furnaces. The mufi le is essentially a refractory enclosure The object to undergo heat treatment is placed inside the mu .e. The mutlie is heated by any suitable means either internally or ex ternally. The advantages of this arrangement are numerous. The mufile serves both as a thermal insulator and reservoir. By virtue of its insulating properties and large heat capacity the mufile evens out any sharp thermal changes that might be caused by draughts, sudden fiuc tuations in heat supply or wnbient temperature.

Furthermore, by proper design of the muiile and heater shapes, the temperature distribution inside the muliie can be accurately controlled. Also a mufiie is indispensable when the heat treatment is to be conducted in an atmosphere other than air.

It is often necessary to visually observe the object inside the mufi'le during heat treatment in order to determine its size, shape, position or color. This is usually accomplished by cutting an opening through the muffle wall to serve as a viewing port. This opening has to be of a size comparable to the object.

Such an opening greatly defeats the chief purpose of the mutlie by introducing a sharp temperature gradient. This gradient is caused by the large heat losses through the opening, which at high temperatures are mainly due to radiation and convection. The introduction or" a transparent heat-resisting window hardly reduces radiation losses, and in cases where the heat treatment is accompanied by volatilization, vapours tend to condense on the relatively cold window rendering it opaque.

According to the present invention, the size of the viewing port is reduced to a minimum whereby heat losses and gradients are completely eliminated without impairing the clearness of observation.

The present invention is based on the fact that an object at high temperature, is in itself a light source. In fact, it is often so highly luminous that it cannot be directly viewed without protection to the eyes.

The muflie and viewing port accordin to this invention, comprise a refractory shell or enclosure provided with an essentially conical hole; the base of the cone being towards the interior of the muffle, facing the object, while its apex reaches slightly beyond the outer mufile wall so as to form a pinhole of such a size as to project a sharp image of the incandescent object on a screen located outside the mutile.

The conical hole is of any general shape, i.c., it is Patented May 3, 1966 conical only in the sense that it is bounded by two surfaces of greatly differing areas. The larger surface faces the object, while the much smaller surface defines a pinhole of a size appropriate to project an image. The position of the conical hole relative to the mufilc and object, as well as the solid angle it subtends, depend solely on the desired field of view, i.e., on the size of the object and its position relative to the pinhole.

The pinhole dimensions are critical. The definition of the image is inversely proportional to those dimensions while the intensity of the image is proportional to their square. The pinhole, however, cannot be made too small because of diffraction effects. The practical pinhole size is determined by the distances of both object and image from the pinhole. it will be shown, hereinafter, that a satisfactory pinhole size can be found in practice.

Optically, the pinhole has the advantage of being free from the aberrations inherent to lenses. By varying the positions of the object and screen relative to the pinhole, an enlarged, reduced or actual size image can be obtained. By attaching a suitable calibrated scale to the screen, the dimensions of the object and its position relative to any desired reference point can easily be measured. Being free from chromatic aberration, the image is an exact colour replica of the object, with the intensity reduced. This offers a means for measuring the temperature of the object, at any corresponding point on the image, by optical pyrometry.

The chief and most important advantage of this pinhole viewing port is that it causes only small heat losses whereby conditions inside the muifie are not disturbed to a large extent by its presence. Due to the smallness of the pinhole, convection and radiation losses are negligible and no temperature gradients are set up in the direction of the pinhole. The object is thus completely thermally isolated. It is therefore practicable to introduce more than one such pinhole, whereby the object might be viewed from several directions, without appreciably affecting the thermal muffle.

A muflle as described above is of particular importance in furnaces used for growing crystals by flame fusion, although the present invention is not restricted to any particular type furnace.

The construction of a conventional flame fusion furnace is well known (see Kirk-Othmer, Encyclopedia of Chemical Technology, vol. 7) and has changed but little since its invention by Verneuil at the turn of the century. In such a furnace an oxyhydrogen burner (some recent setups make use of a plasma torch) discharged its flame vertically into a refractory muffle shaped like a thick walled tube. Fine powder is fed centrally through the flame. The powder melts and collects on top of a re fractory pedestal carrying a seed crystal, and under suitable conditions a single crystal grows from the seed. The crystal grows upwards into hotter zones of the flame. To prevent it from melting completely, it is constantly lowered and kept at a constant distance from the burner, i.e., in a region of constant temperature. This can only be accomplished if the crystal can be observed and its position determined. To this end it is customary to cut an elongated opening through the muffie wall to serve as a viewing port. A suitable sighting device is placed in conditions inside the 3 ventional viewing port. Much heat is lost through the port by radiation and by convection through hot combustion gases.

By using a viewing port and mufile according to this invention, lateral temperature gradients are completely eliminated during both growth and cooling, yielding better and less strained crystals.

The crystal grows as if in a completely closed mufile. Also, determination of the crystal size, shape, position and colour are greatly simplified by attaching suitably calibrated scales on the screen. The use of sighting devices and dark glasses is eliminated.

Other objects of this invention and a fuller understanding of same may be had by referring to the following de scription and claims, taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a schematical perspective view of a muffle according to this invention;

FIGURE 2 is a schematical vertical median cross-sectional view through another mufiie as used in crystal growth by flame fusion;

FIGURE 3 is a schematical perspective horizontal cross-sectional view through another muffle, taken at the plane defined by the axis of the conical hole.

As shown in FIGURE 1, a mufile according to the invention comprises in enclosure 11, provided with a conical recess 12. The base of this recess is at the interior wall of the enclosure while its apex intersects the outer wall defining a pinhole of predetermined diameter. When an object 13 situated inside the enclosure is heated to a high temperature (the heat source 15 is shown schematically in the drawing and it may be situated either inside or outside the enclosure) and thereby becomes incandescent, an inverted image 16 of the object is formed on the screen 17.

A preferred embodiment of the invention is shown in FIGURE 2.

The cylindrical refractory muffle 21 is provided with a cylindrical hole 22 drilled perpendicularly to the mulfie wall. A plug 23 provided with a conical hole 24 fits snugly into 22. The base of the conical hole is at the interior wall of the shell 21 while its apex intersects the outer wall of the plug 23 defining a pinhole 25 of predetermined size. When a crystal 26 supported on the pedestal 2-9 is grown by introducing powder from above through a burner, an upright image 30 of the crystal 26 is obtained on the screen 27 by means of the inverting lens 28. The size and sharpness of the image can be adjusted by changing the relative positions of the crystal, pinhole, lens and screen. It is clearly possible to provide more than one such plug with pinhole at any desired position. Crystals grown in such mufiie show very little internal strain.

It has been found that very satisfactory results can be obtained with the construction shown in FIGURE 3, where -31 is a cylindrical alumina tube surrounded by snugly fitting rings of refractory brick 32. A cylindrical hole 33 passes through 31 and 32. A cylindrical plug 34, made of fired nugget-stone, fits snugly into 33. The plug is provided with a conical recess 35 terminating in a pinhole 36. The growing crystal 37 can be observed on a suitably located screen not shown in the drawing. This cone dimensions and position depend on the dimensions and relative positions of the muflle, burner and crystal. With a cylindrical muilie of outer and inner diameters of 15 cm. and 5 cm. respectively used to grow crystals of up to 2 cm. diameter, a cone of about 25 and a pinhole of 0.05 cm. in diameter yield a sharp 1:1 image which can be clearly seen in an ordinarily lighted room.

Although the invention has been described with a certain degree of particularity, it is understood that the present disclosure is by way of an example only and that many changes in the details of construction, and the combination and arrangement of parts, may be resorted to without departing from the scope and spirit of the invention.

What I claim is:

I. In a device for growing crystals including means to support the crystal, means to feed crystal forming material to said support, means to apply heat to said crystal form ing material to render said material incandescent, and a mufiie surrounding said supporting means to maintain said heat in the area or" said crystal formation, the improvement comprising a viewing port for viewing the interior of said muflle'during said crystal growing without substantially disturbing the thermal conditions within said mufiie, said muffie defining said viewing port as a conical recess with the base of said cone :facing the interior of said muflie and the apex of said cone intersecting the exterior wa l of said mufiie to define a pinhole, and screen means facing said pin-hole at a predetermined distance therefrom to receive an image of said crystal through said viewing port due to the incandescence of said crystal, said pin-hole being of such size as to provide a sharp image on said screen.

2. A mutfle as claimed in claim 1, wherein the size of the pin-hole is between 0.25 to 1.0 mm. in diameter.

3. A muffie furnace as claimed in claim 1, wherein there is provided a suitable optical lens between the pinhole and the screen so as to result in an upright image of the interior of the muffle during its operation.

4-. A device in accordance with claim '1 wherein said muffle comprises a generally cylindrical refractory furnace.

5. A device in accordance with claim 1 wherein said mufiie portion defining said viewing port comprises a cylindrical plug provided with said conical recess, said plug being seated within a cylindrical hole extending through said muill'e.

References Cited by the Examiner UNITED STATES PATENTS 773,202 10/1904 Ewing -64 OTHER REFERENCES The Review of Scientific Instruments, vol. 24, August 

1. IN A DEVICE FOR GROWING CRYSTALS INCLUDING MEANS TO SUPPORT THE CRYSTAL, MEANS TO FEED CRYSTAL FORMING MATERIAL TO SAID SUPPORT, MEANS TO APPLY HEAT TO SAID CRYSTAL FORMING MATERIAL TO RENDER SAID MATERIAL INCANDESCENT, AND A MUFFLE SURROUNDING SAID SUPPORTING MEANS TO MAINTAIN SAID HEAT IN THE AREA OF SAID CRYSTAL FORMATION, THE IMPROVEMENT COMPRISING A VIEWING PORT FOR VIEWING THE INTERIOR OF SAID MUFFLE DURING SAID CRYSTAL GROWING WITHOUT SUBSTANTIALLY DISTURBING THE THERMAL CONDITIONS WITHIN SAID MUFFLE, SAID MUFFLE DEFINING SAID VIEWING PORT AS A CONICAL RECESS WITH THE BASE OF SAID CONE FACING THE INTERIOR OF SAID MUFFLE AND THE APEX OF SAID CONE INTERSECTING THE EXTERIOR WALL OF SAID MUFFLE TO DEFINE A PINHOLE, AND SCREEN MEAN FACING SAID PIN-HOLE AT A PREDETERMINED DISTANCE THEREFROM TO RECEIVE AN IMAGE OF SAID CRYSTAL THROUGH SAID VIEWING PORT DUE TO THE INCANDESCENCE OF SAID CRYSTAL, SAID PIN-HOLE BEING OF SUCH SIZE AS TO PROVIDE A SHARP IMAGE ON SAID SCREEN. 